Xerographic transfer platen



Oct. 25, 1966 c. s. KAISER XEROGRAPHIC TRANSFER PLATEN Filed Jan. 12, 1962 I W W ERASING STATION HIGH VOLTAGE SU PPLY PRINTING DEVELOPING, i 35 STATION STATION INVENTOR. CARL B. KAISER A TTORNEY United States Patent 3,281,857 XEROGRAPi-HC TRANSFER PLATEN Carl B. Kaiser, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N .Y., a corporation of New York Filed .Ian. 12, 1962, Ser. No. 165,891 9 Claims. (Cl. 34674) This invention relates in general to xerography or electrophotography and in particular to transfer of latent electrostatic images from a photosensitive surface to a separable member.

In xerography it is usual to form an electrostatic image on a xerographic plate employing a photoconductive insulating layer. It is also known now to transfer such an electrostatic image to a transfer member or web. (See, for example, application of Harrison Hall assigned to the present assignee, Serial No. 381,890, filed September 23, 1953, now US. Patent 3,084,061, and also US. Patent 2,825,814 to Lewis E. Walkup.) It is advantageous to transfer the electrostatic latent image to a separable transfer member so as to develop the image separately and thus save the photoconductive material from wear. One difficulty with known transfer members is a critical thickness of such a member. It has been found that a transfer member less than two mils thick has a tendency to wrinkle and form excessive and irregular air pockets between it and the adjacent image-bearing member. However, if the transfer member is thick enough to lay well on the image-bearing member and to give desirable ease of handling and durability, the dielectric presented by it attenuates and diffuses the electrostatic field from the electrostatic latent image being transferred. Thus, in the past, it has been necessary to utilize a rather fragile transfer member with a thickness of a few microns in order to attain adequate contrast and resolution.

In accordance with the present invention a transfer member of a convenient weight or thickness may be utilized and at the same time similar field strength and resolution are achieved as with a thin transfer member. This result is accomplished by dispersing conductive particles through the transfer member in such a way as to decrease the effective dielectric thickness by the length of the particles. Also in accordance with the invention the particles may suitably be dispersed in a halftone pattern to produce halftone copy from any original. Thus it is an objective of the present invention to define novel means, methods and apparatus for transfer of electrostatic latent images.

It is an additional objective of the invention to define electrostatic latent image transfer members containing conductive particles dispersed therein.

It is an additional objective of the invention to define an electrostatic image transfer member from which and image forming methods and apparatus in which halftone patterns may be readily reproduced.

Additional objectives of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagrammatic representation of a simple embodiment of apparatus in accordance with the invention;

FIG. 2 is a cross-sectional view of an embodiment of the transfer member in accordance with the invention;

FIG. 3 is a sectional view of xerographic apparatus in accordance with the present invention.

Referring to the drawings wherein reference numerals designate like elements throughout the several figures, FIG. 1 shows an electrostatic latent image-bearing member 11 from which an electrostatic latent image is being transferred to a transfer member 14 by an induction transfer process such as disclosed in the Hall application, Serial No. 381,890. The image-bearing member 11 is depicted as a conventional fiat xerographic plate but is in- 3,281,857 Patented Oct. 25, 1066 tended to represent other members having photoconductive insulating layers as Well as any member with an insulating surface capable of supporting an electrostatic latent image. In accordance with the present invention a transfer member 14, which is relatively thick as compared with prior art, is used without relative deterioration of contrast and resolution in the transferred image. The thick member has enough weight and body to lay well over an image-bearing member without undersirable wrinkles or excessive air pockets between it and the imagebearing member. Thus in FIG. 1, a transfer member 14 in accordance with the invention is positioned over an image-bearing member 11 upon the surface of which an electrostatic latent image has previously been formed by usual methods. cated by the plus signs at the surface of the insulating layer 13. A conductive roller 17 conductively connected by switch 18 to a source of electrostatic charge such as a reference potential (i.e. ground) or to a biasing voltage variable between positive and negative values is caused to traverse the surface of transfer member 14. Transfer member 14 is an insulating sheet containing elongated conductive particles 16. The density of the conductive particles 16 in the member 14 is preferably 2500 per square inch or greater to obtain image reproductions in halftone fineness of lines per inch or better. It is desirable that the conductive particles 16 be exposed for electrical contact on one surface of the transfer member 14 and that their elongated axes are perpendicular to the surface of the transfer member. As the conductive roller 17 traverses the surface of the transfer member 14, it contacts the exposed ends of the conductive particles 16 and allows electrical charges to flow into or out of the conductive particles in response to the electrostatic field of the electrostatic latent image at the surface of the xerographic plate. After the conductive roller has traversed the surface of the transfer member, a charge density varying in accordance with the image pattern will exist over this surface of the transfer member. The transfer member may then be separated from the xerographic plate whereupon potential gradients will appear on the transfer member in accordance with the image pattern and may be developed. Depending on the strength and polarity of the original electrostatic latent image and upon the desired polarity c of the transfer image,

a biasing potential may be placed upon the roller 17 by means of switch 18 and voltage divider 19 including negative and positive voltage sources 20 and 21. One of the advantages that may be derived from biasing in this manner is to control the potential differences that may occur between the surface of the xerographic plate and the transfer member so that deterioration by charge transfer across the separation gap will not occur upon separation of the transfer member from the plate. It is to be understood that the xerographic plate 11 while preferably comprising a conductive backing such as aluminum upon which has been deposited a photoconductive insulator coating such as vitreous selenium, it may be comprised of any photoconductive insulating material and may be in the form of a binder plate or any other plate suitable for use in a xerographic rocess. It is also to be understood that the charging device depicted as roller 17 may also be a flat electrode for contacting the entire transfer member at once, a corona discharge device either AC. or DC, a wiping or brushing material capable of imparting static charge, or an electrically energized electrolytic liquid through which the surface to be charged may be passed.

The transfer member 14, in accordance with the embodiment of FIG. 1, comprises an insulating material 15 such as cellulose acetate, polystyrene, polytetrafluorethylene, polyethylene terephthalate or like resin or plastic material and may be opaque or transparent as desired.

The electrostatic latent image is indiv Dispersed in the insulating material are conductive particles 16 depicted as cylindrical rods but which may be of any shape preferably elongated and may be of any conductive material. These conductive particles may be dispersed in the insulating material 15 by any suitable process such as driving them in, melting the insulating material and mixing the conductive particles therein before shaping the insulating material into the generally flat configuration desired, or by heating a plastic insulating material to an easily previous condition and embedding the particles therein by application of pressure or by virtue of their own weight. Heating the conductive particles themselves is sometimes beneficial. Another process for obtaining a dielectric sheet with insulated rods dispersed in it is to insulate a large quantity of fine wires with a lacquer or resin coating, collate the wires, immerse them in a liquid resin or plastic binder so as to fill all air spaces and then section the collated mass to the desired sheet thickness. After sectioning in the above process, one side of the sheet may be coated with a dielectric material by a deposition process or bybonding to a sheet of dielectric material. Still another method is to puncture a dielectric sheet by electric spark perforation processes to obtain the desired plurality of fine holes. The sheet may then receive a bath in a conductive solution such as a strong salt solution. After the surface of the sheet has been cleaned and the sheet dried, the holes will be conductive. As above, one side of the sheet may then be coated with a dielectric by a deposition process or by bonding to a second sheet of dielectric material.

Arranging the conductive particles in the desired configuration with their axes perpendicular to the surface of the transfer member may be done by any suitable method. Using particles of a ferrous or ferrite material, this configuration may be obtained by miXing the particles in a melted plastic insulating material and then, while the material .is being hardened and formed into the desired flat sheet or web, exposing the insulating material containing the conductive particles to a magnetic field of sufficient strength and in the proper direction to line up the conductive particles as desired. The conductive particles can be exposed on one surface of the transfer member by an abrasion process after the transfer member has hardened.

FIG. 2 shows an embodiment of transfer member 14 in accordance with the invention comprising an insulating material 15 and conductive particles 16. The conductive particles 16 are dispersed in member 14 in an irregular configuration.

In this embodiment of the transfer member, the particles are minute particles having ferromagnetic characteristics. Suitable particles may be carbonyl iron powders, single domain ellipsoidol iron powder or may be any other ferrous or ferrite material or the like including, for example, manganese-antimony-chromium alloys having ferromagnetic characteristics. The particles, having a preferable size of approximately one micron, are mixed in a fluid plastic dielectric binder material. The mixture, in which the particles have a density preferably between 2 and 10 percent, is then subjected to a transverse magnetic field while it is hardened into sheets. Under the influence of the magnetic field the particles form elongated chains perpendicular to the surface of the sheet. While these chains may not be electrically conductive through their lengths, any dielectric remaining between particles in a given chain will be so minute as to be negligible for the purposes of the invention. Formation of transfer members in the above manner permits considerable leeway as to overall thickness of the member.

The present invention has produced greatly improved electrostatic latent images on a transfer member of greater thickness than used heretofore. While not to be in any way limiting, the operation of the invention man occur in accordance with the following theory.

An electrostatic latent image on an insulating surface gives an electrostatic field in accordance with the image d pattern which will attract electrical charges in accordance with the strength of the field in the different points. If an insulating film is placed over such an electrostatic latent image and a source of charge migration is brought to the opposite surface of the insulating film, charges will be attracted to the surface of the insulating film in accordance with the electrostatic field produced by the electrostatic latent image underneath. The quantity of electrical charges that will be attracted is dependent upon the equation Q=CE in which Q represents electrical charges in coulombs, C represents capacitance in farads and E represents potential in volts. Thus the number of charges attracted will be directly dependent upon the voltage difference between the two surfaces of the insulating film and the capacitance controlled by the dielectric presented by the insulating film. In general capacity is determined by the equation E 41rd where K is the dielectric constant, A is the area, and d is the thickness of the dielectric. Thus the thicker the insulating film, the smaller the effective capacitance. The thinner the insulating film, the greater the capacitance. It is thus readily seen that in order to induce a large density of electrical charge migration to the said opposite surface of the insulating film, the insulating film must be kept to a minimum thickness. In accordance with the invention the thickness of an insulating film is effectively reduced wherever the conductive particles are situated since at each point where a conductive particle is contacted with a source of charge migration the end of the conductive particle in closest proximity to the electrostatic latent image becomes effectively one plate of a capacitor, the other plate of the capacitor being the surface at which the electrostatic latent image exists. The effect of the electrostatic latent image field is thus reduced only by the dielectric thickness that exists between the surface of the xerographic plate and the ends of the conductive particles in closest proximity to the surface of the xerographic plate. It is therefore apparent that the use of conductive particles as described permits a heavier, thicker transfer member without affecting the facility with which the image transfer is induced. In practice a transfer member having a physical thickness in the range of 2 to 10 mils and an effective dielectric thickness of 3 to 12 microns has proved satisfactory. These figures are not limiting however, as these dimensions can be varied considerably within the scope of the invention.

FIGURE 3 illustrates a novel xerographic apparatus in accordance with the inventive concept. This novel apparatus avoids the necessity of developing, printing and cleaning of the xerographic plate itself. Developing, printing and cleaning directly on the xerographic plate causes early deterioration in the performance of the xerographic p ate.

In some prior xerographic processes it has been impractical to transfer a latent image from a rotary xerographic plate to an intermediary drum member before developing and printing due to deficiencies in intermediary transfer members available for these processes. For example, prior art transfer members such as those disclosed in the above cited application 381,890 are not readily adaptable to drum use due to their thinness, fragility, lack of resilience and the difficulty of adequately supporting them on a drum. This difiiculty becomes apparent on consideration that no backing is permissible since it would interfere with the application of an electrostatic charge source to the inside surface. The heavier, thicker, more durable and resilient transfer member of the present invention overcomes these past difficulties.

A suitable member as described in relation to the embodiments of FIGURES 1 and 2 herein can readily be constructed in a drum configuration with support at the edges only. Additional required support can be provided at the contacting surfaces by internal rollers. Of particular significance is that the transfer member of the invention may be sufficiently resilient to conform favorably to the surface of the xerographic drum against which it rotates. Accordingly a novel apparatus illustrated in FIG. 3 comprises a drum shaped xerographie plate 11 which is rotated so that its surface passes sensitizing device 31 depicted as a corona discharge device. After sensitizing by device 31 the sensitized surface is exposed to a selective illumination pattern produced by subject 39 conventionally synchronized in movement with the rotation of drum 11 and focused through optical system 37. Under the influence of the selective illumination, charge migration occurs in the plate forming an electrostatic latent image. The latent image bearing plate surface is then rotated synchronously against a rotating transfer drum 14 made of a dielectric containing conductive particles as in the embodiments discussed in relation to FIGURES 1 and 2. A charge pattern of varying density, in conformance to the latent image, is deposited on the transfer drum by roller 17. Roller 17 is maintained at a suitable potential in relation to the backing of xerographic plate 11 by voltage supply 51. As the xerographic plate continues to rotate, it passes erasing device 34, depicted as a source of uniform illumination 35 which disperses the charge patern on the plate. The transfer drum 14 is pressed against the Xerographic plate 11 by roller 17 while the latent image patern is transferred. The latent image on the transfer drum is then carried to developing station 46 Where the image is developed by a usual method such as cascading electroscopic particles over the transfer drum. The developed image is rotated to a printing station 47 where it is then transferred to paper by a suitable method such as rolling paper against the transfer drum under the influence of a corona discharge. After printing, the transfer drum 14 is rotated to an erasing station 48 where the residual printing material and latent image is erased. The erasing station 48 suitably comprises a conductive wiping means to remove electroscopic particles and to dissipate the latent image.

In the drawing a representative segment of xerographic plate 11 is drawn in more detail to show the backing member 12 and the photoconductive layer 13. Likewise the transfer member 14 is represented in greater detail by a representative segment showing insulating material 15 and conductive particles 16.

The operation of roller 17 is worthy of further note. As illustrated, roller 17 is considerably smaller in diameter than the drum members 11 and 14. Due to this difference in diameter, the roller 17 parts contact with a given part of drum transfer member 14 just slightly before drum transfer member 14 parts contact with drum xerographic plate 11. This is necessary, since in the given embodiment the rising potential gradients occurring during the separation of drum members 11 and 14 would be short circuited by roller 17 if roller 17 remained in electrical contact with drum member 14 during said separation. But roller 17 still continues to play an active part in the operation. When drum member 11 and 14 separate, potentials rise and there is a tendency for electrical charges to jump the separating gap causing deterioration of the image. In the embodiment of FIGURE 3 however, the presence of the receding surface of roller 17 reduces this problem substantially. A possible explanation for this may be afforded by considering that the varied charge density occurring on transfer member 14, when drum members 11 and 14 and roller 17 come in contact, is a result of attraction of unlike electrical charges. As the drum surfaces roll apart, the attracted electrical charges are pulled forcibly away from each other causing a strong field tending to restore them to their proximate positions. In the embodiment of FIG- URE 3, however, the separating drum surfaces remain close to backing surfaces comprising roller 17 and backing member 12 which are electrically connected to a common reference point. As the separation gap between the drums increases, the resulting field forces are inductively reflected through the backing members. In the backing members the field forces become distributed over the critical areas of both backing members reducing the spot intensity. In this way separation gap jumping is virtually eliminated.

The present invention lends itself readily to many desirable uses in the art. For example, it is sometimes desirable to reproduce a continuous tone image as a halftone image. As is known in the art, halftone reproduction has certain advantages such as reduction of contrast distortion evidenced by so-called halos and hollow image effects. In accordance with the present invention a halftone reproduction is obtained when the conductive particles in the novel transfer member are dispersed in a halftone pattern.

While the present invention as to its objectives and advantages has been described herein as carried out in specific embodiments thereof, it is not desired to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed it:

1. The method of forming a halftone latent electrostatic image from a continuous tone latent electrostatic image comprising:

(a) positioning a layer of dielectric material containing a halftone dispersion of substantially conductive lines which break only one surface of said layer across a member carrying a continuous tone latent electrostatic image;

(b) inducing a reproduction of said image in said layer by application of an electrical transfer charge that is effective substantially only at the locations of the conductive lines; and,

(c) removing said layer containing said image from said member while avoiding disruptive discharge of the image.

2. The method of claim 1 in which said halftone dispersion has a density of at least 2500 lines per square inch.

3. The method of transferring a latent electrostatic image from an image bearing member comprising:

(a) forming a transfer member by inserting a plurality of elongated electrically conductive particles into a layer of dielectric material having at least a first surface and a second surface and orienting said particles so that they are substantially normal to said surfaces of said layer and break only said first surface of said layer to reduce the effective electrical thickness of said layer,

(b) positioning said transfer member so that said second surface of said layer is in contact with a latent electrostatic image on an image bearing member,

(c) traversing the non-contacting side of the transfer member with a source of electrostatic charge so as to induce a reproduction of said latent electrostatic image on said transfer member, and

(d) removing said transfer member containing said latent electrostatic image from said image bearing member while avoiding dielectric breakdown.

4. The method of forming a halftone latent electrostatic image from a continuous tone latent electrostatic image comprising:

(a) forming a transfer member of a dielectric sheet at least two mils thick and containing a plurality of conductive lines substantially perpendicular to the surface of the dielectric sheet and having a density of said conductive lines of at least 2500 per square inch, said lines so positioned as to break only one surface of said dielectric sheet,

(b) positioning said transfer member across an insulating layer carrying a continuous tone latent electrostatic image,

(c) inducing a transfer of said image by the application of an electrical transfer charge that is effective substantially only at the locations of the conductive lines, and

(d) removing said transfer member containing said image from said insulating layer While avoiding disruptive discharge of the image.

5. The method of transferring a latent electrostatic image from a Xerographic plate comprising:

(a) adding 2 to 10% weight by volume of nonspherical particles of magnetic material to a fiuid dielectric material,

(b) forming said dielectric material into a solid sheet while orienting said particles in chains running substantially normal to the surface of said sheet and partially through the thickness of said sheet,

(c) positioning said sheet against a Xerographic plate carrying a latent electrostatic image,

((1) electrostatically charging the non-contacting side of said sheet so as to induce thereon a reproduction of said image, and

(e) removing said sheet carrying the transferred image from said xerographic plate while avoiding electrical discharge between said sheet and said plate.

6. The method of transferring a latent electrostatic image comprising:

(a) forming a first layer of dielectric material carrying a dispersion of elongated conductive particles traversing its thickness,

(b) laminating to said first layer at least one additional layer of dielectric material electrically insulating the conductive particles on at least one surface of said first layer,

(c) positioning the composite layer formed of said first layer and said second layer in contact with a latent electrostatic image on an insulating surface,

(d) inducing a duplication of said latent electrostatic image in said composite layer by electrostatic induction, and

(e) removing said composite layer containing the transferred image from said insulating surface while avoiding image discharge.

7. Xerographic apparatus comprising:

(a) a xerographic plate,

(b) means to form a latent electrostatic image on said Xerographic plate,

(c) a transfer drum of insulating material containing a dispersion of elongated conductive particles oriented substantially perpendicular to the surface thereof and extending partially through the thickness of said material,

((1) rotational means adapted for rolling said transfer drum against said xerographic. plate, and I (e) means comprising a conductive roller situated internally of said transfer drum, and a reference potential connected to said conductive roller for inducing a reproduction of the latent electrostatic image on the transfer drum while preventing discharge between said transfer drum and said Xerographic plate.

8. Apparatus according to claim 7 in which the dispersion of the conductive particles conforms to the pattern of a halftone screen to form a halftone reproduction of the original image.

9. The method of forming a halftone latent electrostatic image comprising:

(a) forming a transfer member of a dielectric sheet at least two mils thick and containing a halftone pattern of conductive lines oriented substantially normal to the surface of the dielectric sheet, said lines extending partially through the thickness of said sheet,

(b) positioning said transfer member across an insulating layer carrying a latent electrostatic image, and

(c) inducing a reproduction of said image on said member by the application of an electrical transfer charge that is effective substantially only at the locations of the conductive lines.

References Cited by the Examiner UNITED STATES PATENTS 2,584,441 2/ 1952 Fredendall 117--93.2 2,796,359 6/1957 Speed 117-16O 2,862,815 12/1958 Sugarman et a1 117-17.5 2,889,758 6/1959 Bolton 117160 2,916,399 12/1959 Kurz 117-93.2 2,951,247 8/1960 Halpern et al. 117-160 2,992,425 7/1961 Pratt 117-160 3,015,304 1/1962 Carlson et al. 118-637 3,037,478 6/1962 Lace 118-637 3,043,684 7/1962 Mayer a- 117-17.5 3,045,587 7/1962 Schwertz 11717.5

FOREIGN PATENTS 755,486 8/ 1956 Great Britain.

BERNARD KONICK, Primary Examiner.

WILLIAM D. MARTIN, IRVING L. SRAGOW,

' Examiners. P. ROTH, Assistant Examiner. 

1. THE METHOD OF FORMING A HALFTONE LATENT ELECTROSTATIC IMAGE FROM A CONTINUOUS TONE LATENT ELECTROSTATIC IMAGE COMPRISING: (A) POSITIONING A LAYER OF DIELECTRIC MATERIAL CONTAINING A HALFTONE DISPERSION OF SUBSTANTIALLY CONDUCTIVE LINES WHICH BREAK ONLY ONE SURFACE OF SAID LAYER ACROSS A MEMBER CARRYING A CONTINUOUS TONE LATENT ELECTROSTATIC IMAGE; (B) INDUCING A REPRODUCTION OF SAID IMAGE IN SAID LAYER BY APPLICATION OF AN ELECTRICAL TRANSFER CHARGE THAT IS EFFECTIVE SUBSTANTIALLY ONLY AT THE LOCATIONS OF THE CONDUCTIVE LINES; AND, (C) REMOVING SAID LAYER CONTAINING SAID IMAGE FROM SAID MEMBER WHILE AVOIDING DISRUPTIVE DISCHARGE OF THE IMAGE. 